Method and apparatus for carrier recovery and compensation in spread spectrum communications

ABSTRACT

An improved system and method for determining and tracking the carrier offset in a communications system. In an exemplary implementation, sampled spread spectrum digital signals are compensated with an expected carrier offset and stripped of pseudo-noise codes to produce demodulated signals. The demodulated signals are applied to a carrier offset estimator. The carrier offset estimator divides pairs of demodulated signals with a predetermined interval to produce results containing phase change information. The results are then averaged, and the average is normalized with respect to the predetermined interval to yield a new estimate of the carrier offset. The expected carrier frequency is adjusted based on the new offset estimate for frequency compensation in subsequent reception and transmission operations.

BACKGROUND OF THE INVENTION

This invention relates in general to the field of communication systems,and more particularly to carrier recovery and compensation incommunication systems which modulate information with an analog carrierfrequency.

DESCRIPTION OF THE RELATED ART

In a direct sequence spread spectrum communication system, each messagesymbol is spread into a plurality of chips using a pseudo-random (PN)sequence known to both the transmitter and the receiver. The pluralityof chips are then modulated onto an analog carrier signal, such as an RF(radio frequency) signal, for transmission. The receiver down-convertsthe received signals to remove the analog carrier frequency. Ideally, areceiver can perform despreading to remove the PN sequence from thedownconverted received signals with the replica of the PN sequence andrecover the transmitted message symbols. In practice, however, evenafter the analog carrier frequency has been removed, the down-convertedreceived signals, referred to as baseband signals, inevitably stillinclude a carrier frequency. As used herein, the term "analog carrierfrequency" is used to refer to the carrier frequency, such as an RFcarrier frequency, used by an up-converter to modulate data fortransmission over a medium. The term "baseband carrier frequency" orsimply "carrier frequency" is used to refer to the carrier frequencyremaining in the baseband signal after down-conversion has beenperformed. The carrier frequency comprised in the baseband signal is aresult of the frequency difference in the oscillators in the receiverand transmitter. The carrier frequency comprised in the baseband signalis time-varying, and can generally be considered as varying about afixed point, referred to as the expected carrier frequency. The carrierfrequency in the received baseband signal can thus be considered ascomprising an expected carrier frequency plus an unknown carrier offset.

Since the down-converted received signals, or baseband signals, includea carrier frequency, this necessitates the use of carrier recoverybefore regular demodulation of spread spectrum signals can be performed.Conventional carrier recovery methods using analog phase lock loop (PLL)circuits are susceptible to carrier jitter and have fundamentaldifficulties in tracking large carrier variations. A digital carrierfrequency correction approach is disclosed by Stein et al., in "CarrierFrequency Correction for a DSSS Communication System", U.S. Pat. No.5,303,257, April/1994. The method provides robust carrier offsetestimation by computing the energy of received signal in a plurality offrequency bins. One drawback however, is that the resolution of thecarrier estimate is limited by the number of frequency bins involved.Hence, in general the carrier compensation is sub-optimal even withoutthe presence of noise or interference.

Therefore, a system and method is desired to accomplish precise carrieroffset estimation with fixed low computations without sacrificing therobustness of spread spectrum communications.

SUMMARY OF THE INVENTION

The present invention comprises a system and method for performing moreprecise carrier offset estimation with reduced computations. In apreferred spread spectrum embodiment of the invention, during a trainingperiod pilot signals modulated by the respective user's PN sequence aretransmitted to provide a carrier reference for the receiver. The pilotsignals are sampled and decimated at the receiver to produce chip-ratedigital spread spectrum signals. The digital spread spectrum signalscontain information regarding the baseband carrier frequency, referredto simply as the carrier frequency. The carrier frequency can bedecomposed into an expected carrier frequency and a carrier frequencyoffset. Mathematically, the digital spread spectrum signals can berepresented as

    x(k)=p(k)e.sup.jk(ω.sbsp.0.sup.+δω.sbsp.0.sup.)

where p(k) denotes the PN code sequence, ω₀ is the expected carrierfrequency, and δω₀ represents the unknown carrier frequency offset to beestimated. The present invention provides an improved system and methodfor estimating and compensating the undesirable carrier offset based ona finite number of data samples: x(k), k=1,. . . , K. Thus the presentinvention determines and compensates for the carrier frequency.

In accordance with the present invention, the digital spread spectrumsignals are first stripped of the PN sequence and compensated with theexpected carrier frequency provided by a local digital oscillator. Theresulting demodulated received signals, y(k)=e^(jk)(δω.sbsp.0.sup.),k=1, . . . , K, are then applied to a carrier offset estimator todetermine the unknown carrier offset. The system and method of thepresent invention computes a result containing phase change informationfor pairs of the values y(k), k=1, . . . , K, and averages theseresults, to obtain a precise estimate of the carrier offset. Asmentioned above, the carrier frequency offset estimation receivesdemodulated received signals referred to as y(k)=e^(jk)(δω.sbsp.0.sup.),k=1, . . . , K.

The estimator first computes a result containing phase changeinformation for pairs of the demodulated received signals y(k). Each ofthe pairs comprises two values of y(k) having different values of k. Inother words, each of the pairs comprises two elements, wherein eachelement of a pair is value of y(k) having a respective value of k.Computing a result containing phase change information preferablycomprises dividing elements of pairs of the demodulated receivedsignals, i.e., dividing different values of y(k), to compute a resultcontaining phase change information for each of the pairs.Alternatively, The computing comprises multiplying a first element of apair with the conjugate of the other element of the pair. The resultcontaining phase change information is computed for at least a subset ofthe total possible pairs of the demodulated received signals.

In one embodiment, the computing comprises computing a result containingphase change information for at least a subset of the neighboring pairsof y(k). In another embodiment, the system computes a result containingphase change information for pairs of the demodulated received signalsy(k), wherein elements of the pairs have one or more predeterminedL-chip or L-sample intervals between the elements of the pairs. Afterthe results are computed, the system calculates the average of at leasta subset ofthe results. The system then normalizes the average, ifnecessary, to compute the carrier offset estimate. The normalization isperformed using the one or more L-chip or L-sample intervals used in thecomputing.

After the carrier frequency offset estimation is performed, the systemthen adjusts the expected carrier frequency based on the carrier offsetestimate. The adjusted expected carrier frequency is used for subsequentreceived digital spread spectrum signals to demodulate the subsequentreceived digital spread spectrum signals to remove the carrierfrequency.

Before the expected carrier frequency is adjusted based on the carrieroffset estimate, the system preferably performs validity testing toexamine the validity of the carrier offset estimate. Thus the adjustmentis only made using valid carrier offset estimates. The validity test mayinvolve examining variations of the current carrier offset estimate withrespect to a plurality of prior carrier offset estimates and determiningif the variations are greater than a pre-determined threshold.Alternatively, the validity test may involve determining a carrieroffset estimate using the Fourier Transform of the demodulated receivedsignals to produce a reference carrier offset estimate, comparing thecarrier offset estimate to the reference carrier offset estimate, anddetermining if the carrier offset estimate is within a pre-determinedrange of the reference carrier offset estimate. Finally, the validitytest may involve examining the signal-to-noise ratio (SNR) of thedemodulated received signals and determining if the SNR is below apre-determined threshold.

According to the present invention, substantially exact carrierfrequency offset estimation can be achieved in the absence of noise. Thepresent invention provides an efficient estimate of the carrier offsetwithout a large amount of computations. The present invention also hasimproved reliability and reduced storage requirements over prior artsystems. The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate particular embodiments of theinvention, and together with the description, serve to explain theprinciples of the invention.

FIG. 1 illustrates a functional diagram of a typical spread spectrumcommunication system;

FIG. 2 is a block diagram of an embodiment of a carrier offsetestimation system in accordance with the present invention;

FIG. 3 is a flowchart diagram illustrating one embodiment of carrierrecovery and compensation operations in accordance with the presentinvention;

FIG. 4 is a flowchart diagram illustrating operation of the carrieroffset estimator of FIG. 2; and

FIG. 5 is a block diagram of an embodiment of a carrier offset estimatorin accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Incorporation byReference

The following U.S. Patents are hereby incorporated by reference asthough fully and completely set forth herein.

U.S. Pat. No. 4,336,616 to Carson et al. June/1982

U.S. Pat. No. 4,630,283 to Schiff December/1986

U.S. Pat. No. 4,841,544 to Nyutkens January/1989

U.S. Pat. No. 5,291,517 to Stein et al. May/1994

U.S. Pat. No. 5,303,257 to Stein et al. April/1994

U.S. Pat. No. 5,329,546 to Lee July/1994

U.S. Pat. No. 5,359,624 to Lee et al October/1994

FIG. 1 illustrates a functional diagram of a spread spectrumcommunication system including a carrier estimation system 6 accordingto the present invention. In the preferred embodiment, the carrierestimation system 6 is comprised in a spread spectrum communicationsystem. However, it is noted that the carrier estimation system 6 of thepresent invention may be incorporated into any of various systems,including both spread spectrum (SS) and non-spread spectrum systems,which either modulate information on an analog carrier wave or otherwiseare required to analyze a baseband signal which includes an unknowncarrier frequency that must be compensated. For more information on thesystem of the preferred embodiment, please see related co-pendingapplication Ser. No. 08/768,100 filed Dec. 16, 1996, titled "SmartAntenna CDMA Wireless Communications System", whose inventors are HuiLiu and Guanxghan Xu, and which is hereby incorporated by reference inits entirety as though fully and completely set forth herein.

The system of FIG. 1 is preferably implemented using one or moreprogrammable digital signal processors (DSPs) and one or more memories.However, the system may be implemented in various manners, including oneor more programmable CPUs, microcontrollers or other programmable logic,discrete logic, etc.

In the embodiment shown, time-division-duplexing (TDD) is adopted fortransmitted and received data, meaning that transmission (TX) andreception (RX) are performed in separate time frames. However, it isnoted that any of various transmission and reception schemes may beused.

As shown, the spread spectrum communication system includes an antenna10 which is adapted for transmitting and receiving signals modulatedwith an analog carrier frequency, preferably RF (radio frequency)modulated signals. The antenna 10 is coupled to a down-converter 12. Thedown-converter 12 down-converts the received signal from radio frequency(RF) to baseband. In other words, the down-converter 12 operates todemodulate the information from the analog carrier wave, i.e., to removethe analog carrier frequency.

The down-converter 12 is coupled to provide its output to a sampler 16.The sampler 16 receives the baseband signal and performs analog todigital conversion to produce digital spread spectrum signals 40. Thesampler 16 preferably oversamples the signal and then decimates thesamples to produce chip rate signals. The sampler 16 provides thedigital spread spectrum signals or chip rate signals 40 to the carrierestimation system 6 according to the present invention. The sampler 16receives an output from a timing recovery module 28, as shown. Theinformation from the timing recovery module is used during thedecimation process to correctly produce the chip rate signals.

The carrier estimation system 6 operates to estimate the carrierfrequency still contained in the down-converted received signals 40,also referred to as the baseband signals 40. As mentioned above, thedown-converter 12 operates to remove the analog carrier frequency fromthe received signals. However, due to the frequency difference betweenthe two oscillators in the transmitter and the receiver, and possibleinaccuracies in the transmitter modulator, the down-converted receivedsignals or baseband signals 40 still include an unknown baseband carrierfrequency, referred to herein as the carrier frequency.

As used herein, the term "analog carrier frequency" is used to refer tothe carrier frequency, such as an RF carrier frequency, used by anup-converter to modulate data for transmission over a medium. The term"baseband carrier frequency" or simply "carrier frequency" is used torefer to the carrier frequency remaining in the baseband signal afterdown-conversion has been performed. The carrier frequency comprised inthe baseband signal is a result of the frequency difference in theoscillators in the receiver and transmitter. The carrier frequencycomprised in the baseband signal is time-varying, and can generally beconsidered as varying about a fixed point or "guess", referred to as theexpected carrier frequency. The carrier frequency in the receivedbaseband signal can thus be considered as comprising an expected carrierfrequency plus an unknown carrier offset. The carrier frequency offsetthus must be estimated, and the total carrier frequency comprising theexpected carrier frequency and the carrier frequency offset must becompensated for improved results.

The carrier estimation system 6 provides the received signals, with theexpected carrier frequency and the carrier frequency offset removed, toa spread spectrum (SS) demodulator 24 for demodulation. The SSdemodulator 24 operates to recover the transmitted message data from theremote terminal as is done in the current art. The carrier estimationsystem 6 also provides an output comprising the total estimated carrierfrequency to a timing recovery module 28 and a mixer 64 in thetransmission path.

The timing recovery module 28 receives an output from the carrierestimation system 6 and also receives the signals 40 output from thesampler 16. The timing recovery module 28 operates to determine a timingsignal based on the carrier offset frequency. The timing recovery module28 provides this timing signal to the sampler 16 and to a timingadjuster 18 in the transmission path. The timing signal is used by thesampler 16 to aid in the decimation process to produce the chip ratesignals 40, as described above. The timing signal is also used by thetiming adjuster 18 in the transmission path for similar purposes.

The transmission path comprises an SS modulator 26 which modulatesmessage data with a pseudo noise signal or spread spectrum signal. TheSS modulator 26 modulates message data intended for remote terminals asis done in current art. A pulse shaping module 22 is coupled to theoutput of the SS modulator 26 and operates to limit signal energy to apredetermined bandwidth. The pulse shaped signals are then provided to amixer 64. The mixer 64 operates to mix the pulse shaped signals with acarrier frequency. As described above, the mixer 64 receives the carrierfrequency from the carrier estimation system 6, wherein the carrierfrequency comprises the expected carrier frequency and the estimatedcarrier frequency offset. The mixer 64 modulates the signals with thecarrier frequency determined by the carrier offset estimator 38 topre-compensate the transmitted signal with the difference in oscillatorfrequencies.

The mixer 64 provides its output to a timing adjuster 18. As mentionedabove, the timing adjuster 18 also receives a timing signal from thetiming recovery module 28. The timing adjuster 18 adjusts the transmittiming based on the information provided by the timing recovery module28. The resulting baseband signal is then applied to an up-converter(14). The up-converter 14 modulates the baseband signal with the analogcarrier frequency, and the resulting signals are provided to the antenna10 for transmission.

Therefore, the mixer 64 operates to modulate the message information, inthis embodiment the spread spectrum modulated signals, with both theexpected carrier frequency and the carrier frequency offset. Theup-converter 14 further operates to modulate the baseband messageinformation with an analog carrier frequency for transmission.

Therefore, during a first time frame, i.e., the reception frame, theantenna 10 receives a signal and the down-converter 12 down-converts thereceived signal from radio frequency (RF) to baseband. The sampler 16digitizes the baseband signals to produce digital spread spectrumsignals 40. As mentioned above, even after down-conversion, the basebandsignals are still modulated by an expected carrier frequency, which isknown, and a carrier frequency offset, which is unknown. The carrierestimation system 6 estimates the carrier frequency offset and providesthe expected carrier frequency source for both the timing recoverymodule 28 and the mixer 64 in the transmission path.

During a second time frame, i.e., the transmission frame, message dataintended for remote terminals are modulated by the SS modulator 26, andpulse shaping is then performed. The pulse shaped signals are fed to themixer 64 to mix with the carrier frequency provided by the carrierestimation system 6. The resulting signals are then provided through thetiming adjuster 18, and the baseband signal is then provided to theup-converter 14. The up-converter 14 modulates the baseband signal on ananalog carrier frequency, and the resulting signal is provided to theantenna 10 for transmission.

In spread spectrum communications, a communication from each userinitially begins with a training period followed by actual voice ormessage data. During the training period, pilot signals modulated by therespective user's PN sequence are transmitted to provide a carrierreference for the receiver. The pilot signals are sampled and decimatedat the receiver to produce chip-rate digital spread spectrum signals.The digital spread spectrum signals contain information regarding thecarrier frequency. The carrier frequency can be decomposed into anexpected carrier frequency and a carrier frequency offset.Mathematically, the digital spread spectrum signals can be representedas

    x9k)=p(k)e.sup.jk(ω.sbsp.0.sup.+δω.sbsp.0.sup.)

where p(k) denotes the PN code sequence, ω₀ is the expected carrierfrequency, and δω₀ represents the unknown carrier frequency offset to beestimated. The objective herein is to estimate and compensate theundesirable carrier offset based on a finite number of data samples:x(k), k=1, . . . , K.

Briefly, in accordance with the present invention, the digital spreadspectrum signals are first stripped of the PN sequence and compensatedwith the expected carrier frequency provided by a local digitaloscillator 34 (FIG. 2). The resulting demodulated received signals,y(k)=e^(jk)(δω.sbsp.0.sup.), k=1, . . . , K, are then applied to acarrier offset estimator 38 (FIG. 2) to determine the unknown carrieroffset. Prior art methods generally estimate δω₀ by mixing y(k) withpre-stored samples of a plurality of closely spaced sinusoidal tones. Incontrast to these prior art methods, the disclosed approach calculatesphase change information of y(k), k=1, . . . , K, to obtain a preciseestimate of the carrier offset. The calculation of phase changeinformation preferably involves dividing elements of the demodulatedreceived signals with each other to compute a result containing phasechange information for each of the at least a subset of pairs. Themethod then calculates the average of at least a subset of the resultsto compute the carrier offset estimate, and then normalizes asnecessary.

The system and method of the present invention provides substantiallyexact carrier frequency offset estimation in the absence of noise.Robustness is provided by a validity tester which excludes invalidestimates. Only valid estimates are passed along to an adjuster whichupdates the expected carrier frequency for demodulation of subsequentreceived digital signals.

FIG. 2--Carrier Estimation System

FIG. 2 shows a block diagram of one embodiment of the carrier estimationsystem 6 in accordance with the present invention. As noted above, thecarrier estimation system 6 may be implemented in any of various ways,including one or more DSPs and associated one or more memories,programmable CPUs microcontrollers or other programmable logic, discretelogic, analog circuitry, or combinations thereof.

Techniques well-known in the art can be used to down-convert thereceived signals to baseband and perform sampling and synchronization togenerate samples of the received signal. Inputs to the present system,x(k), are digital signals. As noted above, the analog carrier frequencyhas already been removed from the digital signals x(k). The digitalsignals x(k) include a carrier frequency which can be considered ascomprising an expected carrier frequency and a carrier frequency offset.In the preferred spread spectrum embodiment, the inputs x(k) are digitalspread spectrum signals. As noted above, the inputs x(k) may compriseother types of digital signals, e.g., non-spread spectrum signals.

As shown, the carrier estimation system 6 comprises an oscillator 34coupled to a multiplier or mixer 33. The inputs x(k) are provided to aninput of the multiplier 33. The multiplier 33 operates to mix thedigital spread spectrum signals with the expected carrier frequencyprovided by the digital oscillator 34. The multiplier 33 thus operatesto remove the expected carrier frequency from the inputs x(k).

It is noted that the preferred embodiment of the invention operates toremove the expected carrier frequency and then determine the remainingoffset. In another embodiment, the system does not remove an expectedcarrier frequency, but rather determines the entire unknown carrierfrequency.

The output of the multiplier 33 is provided to a second mixer 32. A PNcode generator 30 is coupled to provide a replica of the PN sequence tothe multiplier 32. The multiplier 32 multiplies the received signalswith the replica of the PN sequence to further remove the PN codesequence. Thus the multipliers 33 and 32 operate to remove the expectedcarrier frequency and the PN code sequence from the inputs x(k) . It isnoted that the multiplier 32 and the PN code generator 30 are includedin the spread spectrum preferred embodiment, but may be omitted innon-spread spectrum embodiments.

The resulting demodulated received signals y(k) 42 are applied to acarrier offset estimator 38 for estimating the carrier frequency offsetduring the training period. After the training period, the demodulatedreceived signals are passed along to the SS demodulator 24 to recoverthe transmitted message data.

According to the present invention, the carrier offset estimator 38provides an accurate carrier offset estimate 44 from the demodulatedreceived signal, y(k). Detailed discussion regarding the frequencyestimation mechanism is provided below. In the preferred embodiment thecarrier offset estimator 38 is coupled to a validity tester 46 whichexamines the validity of the estimate. Upon completion of the carrieroffset estimation by the carrier offset estimator 38, the estimatedcarrier offset is provided to the validity tester 46. It is noted thatvalidity testing is preferably performed, but may be omitted, asdesired.

A variety of approaches can be employed to examine whether or not theestimate is qualified to update the expect carrier frequency. Here theterm δω₀ (n) is used to denote the carrier offset estimate obtainedduring the nth reception frame. In one embodiment, the current carrieroffset estimate is compared with its previous values. Since onlygraceful carrier variations are possible in practice, any estimate witha relative change, i.e., |δω₀ (n)-δω₀ (n-k)|, k=1, . . . , , over apredetermined threshold is excluded.

In another embodiment, the sequence of carrier offset estimates ispassed through a low pass FIR filter to smooth out estimates with highvariations. In another embodiment, a frequency domain transform,preferably an FFT, is applied to the demodulated received signals toobtain a reference carrier offset estimate, and the carrier offsetestimate is determined to be invalid if the carrier offset estimate isnot within a pre-determined range of the reference carrier offsetestimate. Yet in another embodiment, the signal-to-noise ratio (SNR) ofthe demodulated received signals is first examined. The carrier offsetestimate is determined to be invalid if the SNR is below apre-determined threshold. Any of various methods may be used forvalidity testing, as desired.

The validity tester 46 is coupled to a carrier adjuster 36. Once a validcarrier offset estimate, denoted hereafter as δω, is identified, thecarrier adjuster 46 updates the expected carrier frequency. In oneembodiment, the carrier adjuster 36 simply modifies the new expectedcarrier frequency as ω₀ (n)=ω₀ (n-1)+δω₀ (n). In another embodiment, thecarrier adjuster 36 accounts for additional information, i.e., theconfidence level of the carrier offset estimate, and updates theexpected carrier frequency more gracefully. More sophisticatedtechniques well-known in the art can be employed to realize the samefunction.

FIG. 3--Carrier Estimation System Method

FIG. 3 is a flowchart diagram illustrating the disclosed carrier offsetestimation operations, i.e., operation of the carrier estimation system6, in accordance with the present invention.

As shown, in step 102 the system receive digital signals. In thepreferred spread spectrum embodiment, the received digital signalscomprise digital spread spectrum signals. The digital spread spectrumsignals are generated using analog circuits and samplers as is done inthe current art. In step 104 the system removes the expected carrierfrequency from the received signals. Thus, in order to recover theunknown carrier offset in the digital signals, the expected carrierfrequency is first demodulated or removed. In step 106 the systemunmasks the PN code sequence from the spread spectrum digital signals.It is noted that step 106 is performed only in the preferred spreadspectrum embodiment. Thus, in order to recover the unknown carrieroffset in received digital spread spectrum signals, the PN sequence andthe expected carrier frequency are both first demodulated or removed.

After operation of steps 102-106, the resulting demodulated receivedsignals are referred to as y(k)=e^(jk)(δω.sbsp.0.sup.), k=1, . . . , K.These values are then applied to the carrier offset estimator 38 todetermine the unknown carrier offset. If there were no carrier offset,then the signals y(k) would comprise a DC value. However, due to thecarrier offset, the signals y(k) essentially comprise a sinusoid.

In step 108 the system performs carrier offset estimation. As discussedfurther below, in the preferred embodiment a plurality of pairs of theresulting demodulated received signals are divided to produce a set ofresults containing phase change information. The results are thenaveraged, and normalization is optionally performed, to produce thecarrier offset estimate. In step 110 validity testing is preferablyperformed to evaluate the "goodness" or accuracy of the estimate. Thevarious different types of validity testing were discussed above. Instep 112 the system adjusts the carrier frequency if the carrierfrequency offset estimate is deemed valid. Thus, only valid carrieroffset estimates are preferably used in adjusting the expected carrierfrequency for demodulating subsequent signals.

FIG. 4--Carrier Offset Estimator

Referring now to FIG. 4, a flowchart diagram illustrating the carrieroffset estimation performed in step 108 of FIG. 3 is shown. FIG. 4illustrates the operation of the carrier offset estimator 38 of FIG. 2.As mentioned above, the carrier frequency offset estimation receivesdemodulated received signals referred to as y(k)=e^(jk)(δω.sbsp.0.sup.),k=1, . . . , K.

As shown, in step 122 the estimator 38 computes a function of the phasechange for a plurality of pairs of the received demodulated signals. Inthe preferred embodiment, the estimator 38 divides pairs of thedemodulated received signals y(k) to compute a result containing phasechange information for each of the pairs. Each of the pairs comprisestwo values of y(k) having different values of k. Each of the pairs canbe stated to comprise two elements, wherein each element of a pair isvalue of y(k) having a respective value of k. The dividing comprisesdividing elements of pairs of the demodulated received signals, i.e.,dividing different values of y(k), to compute a result containing thephase change information for each of the pairs. The dividing preferablycomprises dividing elements of pairs of the demodulated received signalsfor at least a subset of the total possible number of pairs. It is notedthat the dividing operation can be realized by multiplying a firstelement with the conjugate of the second element.

In one embodiment, the dividing comprises dividing at least a subset ofthe neighboring pairs of y(k), i.e., pairs comprising neighboring valuesof y(k), such as {y(1), y(2)}, {y(2), y(3)}, {y(3), y(4)}, etc.

In another embodiment, dividing comprises dividing at least a subset ofpairs of the demodulated received signals y(k), wherein elements of thepairs have a predetermined L-chip interval between the elements of thepairs. An example of this embodiment is a sequence of pairs such as{y(1), y(4)}, {y(2), y(5)}, {y(3), y(6)}, etc.

In another embodiment, dividing comprises dividing at least a subset ofpairs of the demodulated received signals y(k), wherein elements of thepairs have two or more differing predetermined L-chip intervals betweenthe elements of the pairs. An example of this embodiment is a sequenceof pairs such as {y(1), y(2)}, {y(2), y(3)}, {y(3), y(6)}, {y(4), y(7)},etc. In this embodiment, normalization is required for each group ofpairs that uses a different predetermined L-chip interval.

After dividing is performed in step 122, in step 124 the systemcalculates the average of at least a subset of the results containingphase change information to compute the carrier offset estimate. In thepreferred embodiment, all of the computed results are used. In anotherembodiment, a subset of these results is used, e.g., one or more of thelow and high results are thrown out, and the remaining results are usedin the averaging process.

After averaging is performed in step 124, in step 126 the systemnormalizes the average, if necessary, to compute the carrier offset. Thenormalization is performed using the one or more L-chip intervals usedin the dividing. In an embodiment where the dividing is performed solelyusing neighboring pairs of y(k) values, the L-chip interval is 1, and nonormalization is necessary. In an embodiment where the dividing isperformed using pairs have a single predetermined L-chip intervalbetween the respective elements of the pairs of y(k) values, thenormalizing is performed using a factor of L to compensate for theL-chip interval.

As mentioned above, where the dividing includes dividing the at least asubset of pairs of y(k) with one or more predetermined chip intervals,the normalizing is performed for each respective group using respectiveones of the one or more predetermined chip intervals, and thisnormalization is performed prior to averaging. Alternatively, the systemcan first calculate one or more averages for each of the respectivegroups which uses a different predetermined chip interval. This isfollowed by normalizing each of the one or more averages by respectiveones of the one or more predetermined chip intervals. This normalizingproduces one or more normalized averages. The system then averages theone or more normalized averages to compute the carrier offset estimate.

In a non-spread spectrum embodiment, the dividing comprises dividing atleast a subset of pairs of the demodulated received signals y(k),wherein elements of the pairs have one or more differing predeterminedtime intervals between the elements of the pairs. Similar embodiments tothose described above may also be used, where differing one or more timeintervals are used instead of differing one or more L-chip intervals.

In the preferred embodiment, in the initial iteration of the flowchartof FIG. 4, the expected carrier frequency is initially set to 0, and thefirst carrier frequency offset estimate is the entire determinedbaseband carrier frequency. On the next iteration, this initial carrierfrequency offset estimate is then used as the expected carrierfrequency, and a new offset is determined based on this expected carrierfrequency. On subsequent iterations, the combined expected carrierfrequency and carrier frequency offset estimate determined in arespective calculations are used as the next expected carrier frequencyin the subsequent calculation. The flowchart thus proceeds adaptively inthis manner, i.e., the expected carrier frequency is adjusted based onthe new offset estimate for frequency compensation in subsequentreception and transmission operations.

FIG. 5--Carrier Offset Estimator Embodiment

FIG. 5 illustrates an exemplary embodiment of the carrier offsetestimator 38 in accordance of the present invention. Demodulated signals(22), y(k), are piped to a switch 50 and distributed to a first register52 and a second register 54 for storing respective elements of pairs ofthe demodulated received signals with time interval L. Thus one register52 stores a value y(k1), and the second register 54 stores a value y(k2)

A divider 56 is coupled to the first and second registers 52 and 54 fordividing an element of a pair stored in the first register by an elementof the pair stored in the second register to produce a result containingthe phase change information. An averager 58 is coupled to the divider56 for calculating an average of results to compute the carrier offsetestimate. Finally, a normalizer 60 is coupled to the averager 58 fornormalizing the average to compute the carrier offset estimate.

As noted above, the carrier offset estimator 38 may be implemented inany of various manners, including one or more DSPs and associated one ormore memories, programmable CPUs microcontrollers or other programmablelogic, discrete logic, analog circuitry, or combinations thereof.

It is noted that only registers of length L are required to buffersamples with time separation of L chips. The averager 58 can beimplemented adaptively and thus requires no storage. The capability ofobtaining a high-resolution carrier offset estimate with minimum storageand computations is unique to the present invention.

In an ideal scenario, the demodulated received signal is simply a puretone which implies that any pair of samples contains sufficientinformation of the carrier offset. In practice however, noise and otherpeturbation is inevitable. Thus a plurality of pairs of samples areused, with possible differing L-chip or time intervals. The use of aplurality of pairs of samples, followed by averaging and normalization,tend to cancel out the noise and peturbations.

The following summarizes the preferred embodiment of the estimationprocedure,

Divide (or multiply a conjugate of one element with another element) aplurality of pairs of samples with a predetermined time interval, L, toobtain a set of results containing phase change information: ##EQU1##where n(k) denotes the peturbation term; Calculate the average of theresulting set of estimates: ##EQU2## Estimate the carrier offset as thenormalized (by a factor of L) phase of the average.

As noted above, in one embodiment, L is simply chosen to be 1, i.e.,only neighboring elements are used in computation. The carrier offsetestimate can thus be accomplished without normalization. In anotherembodiment, pairs of demodulated received signals with one or morepredetermined chip intervals or time intervals are first divided toproduce a set of results containing the phase change information; eachof the results are then normalized with respect to its correspondinginterval to produce a set of normalized results; and finally, thenormalized results are averaged to produce a carrier offset estimate. Inyet another embodiment, pairs of the demodulated received signals withone or more predetermined chip intervals are divided to compute a set ofresults containing the phase change information; each subset of resultswith the same interval are averaged; and all averages are thennormalized with respect to their respective intervals to produced a setof normalized results; and the set of normalized results are thenaveraged to produce a carrier offset estimate.

Evidently, the computations required to accomplish the above estimationis fixed in each embodiment, yet the resolution of carrier offsetestimate has no limit. The present invention addresses both accuracy andcost issues and clearly has significant advantages over priorapproaches. In the preferred embodiment, L is selected to be greaterthan 1. The reason to select L other than 1 in the preferred embodimentis two-fold: First of all, by increasing the phase difference from δω₀to Lδω₀, the phase of z can be more easily detected, thereby allowingmore accurate estimation of the true carrier offset, δω₀ ; Moreimportantly, the expansion in phase provides additional resistance toperturbation, and hence leads to more reliable estimation.

Some of the advantages of the present invention may be summarized asfollows:

The disclosed method provides an efficient estimate of the carrieroffset by exploiting the structure information of the demodulatedreceived signals. Contrasting to prior art where high resolution canonly be achieved at the cost of computations, the present method hasfixed computational expense without resolution or estimation rangelimitations.

Reliability is guaranteed by means of validity testing. The expectedcarrier frequency is adjusted adaptively using only valid carrier offsetestimates, thereby eliminating the possibility of catastrophic errors.

No storage requirements: In the prior art, sufficient time and memorymust be allocated to construct and store samples of closely spacedsinusoidal tones; this may become exceedingly different as theresolution requirement increases. The estimation scheme in the presentinvention has minimum requirements in both memory and computationaltime.

It will be apparent to those skilled in the art that variousmodifications can be made to the carrier recovery and compensationsystem and method of the instant invention without departing from thescope or spirit of the invention, and it is intended that the presentinvention cover modifications and variations of the carrier recovery andcompensation system and method provided they come in the scope of theappended claims and their equivalents.

We claim:
 1. In a direct sequence spread spectrum communication system,wherein digital spread spectrum signals modulated by a pseudo-randomsequence have a carrier frequency, wherein the carrier frequencyincludes an expected carrier frequency and a carrier frequency offset, acarrier offset estimation method for recovering the carrier offset,comprising the steps of:receiving digital spread spectrum signalsmodulated with a pseudo-random sequence, wherein said digital spreadspectrum signals have a carrier frequency, wherein the carrier frequencyincludes an expected carrier frequency and a carrier frequency offset;demodulating said digital spread spectrum signals to remove saidexpected carrier frequency, wherein said demodulating also includesdemodulating said digital spread spectrum signals to remove saidpseudo-random sequence, wherein said demodulating produces demodulatedreceived signals; computing a result containing phase change informationfor each of a plurality of pairs of said demodulated received signalswherein elements of said plurality of pairs have a predetermined L-chipinterval; calculating the average of said results to compute the carrieroffset estimate; and normalizing said average by a factor of L tocompute the carrier offset estimate, wherein said normalizing isperformed after said calculating said average.
 2. The carrier offsetestimation method of claim 1, further comprising:adjusting said expectedcarrier frequency based on said carrier offset estimate, wherein saidadjusted expected carrier frequency is used for subsequent receiveddigital spread spectrum signals to demodulate said subsequent receiveddigital spread spectrum signals to remove said carrier frequency.
 3. Thecarrier offset estimation method of claim 2, furthercomprising:examining the validity of said carrier offset estimate aftercomputing said carrier offset estimate; wherein said adjusting isperformed using only valid carrier offset estimates.
 4. The carrieroffset estimation method of claim 3, wherein said examining the validityof said carrier offset estimate includes:examining variations of thecurrent carrier offset estimate with respect to a plurality of priorcarrier offset estimates; determining if said variations are greaterthan a pre-determined threshold, wherein said carrier offset estimate isdetermined to be invalid if said variations are greater than saidpre-determined threshold.
 5. The carrier offset estimation method ofclaim 3, wherein said examining the validity of said carrier offsetestimate includes:determining a carrier offset estimate using theFourier Transform of said demodulated received signals to produce areference carrier offset estimate; comparing said carrier offsetestimate to said reference carrier offset estimate; determining if saidcarrier offset estimate is within a pre-determined range of saidreference carrier offset estimate, wherein said carrier offset estimateis determined to be invalid if said carrier offset estimate is notwithin said pre-determined range of said reference carrier offsetestimate.
 6. The carrier offset estimation method of claim 3, whereinsaid examining the validity of said carrier offset estimateincludes:examining the signal-to-noise ratio (SNR) of said demodulatedreceived signals, wherein said carrier offset estimate is determined tobe invalid if said SNR is below a pre-determined threshold.
 7. Thecarrier offset estimation method of claim 1,wherein each of said pairscomprises two elements; wherein said computing a result comprising phasechange information comprises dividing elements of each of said pluralityof pairs of said demodulated received signals to compute a resultcomprising phase change information for each of said plurality of pairs.8. The carrier offset estimation method of claim 1,wherein each of saidpairs comprises first and second elements; wherein said computing aresult comprising phase change information for a pair of saiddemodulated received signals comprises multiplying a first element ofthe pair with a conjugate of the second element of the pair to computethe result comprising phase change information for the pair.
 9. Acarrier offset estimation system for recovering carrier offset in adirect sequence spread spectrum communication system, wherein thecommunication system receives digital spread spectrum signals modulatedwith a pseudo-random sequence, wherein the digital spread spectrumsignals have a carrier frequency, wherein the carrier frequency includesan expected carrier frequency and a carrier frequency offset, thecarrier offset estimation system comprising:a demodulator fordemodulating said digital spread spectrum signals to remove saidexpected carrier frequency, wherein said demodulator also demodulatessaid digital spread spectrum signals to remove said pseudo-randomsequence, wherein said demodulator produces demodulated receivedsignals; a carrier offset estimator, comprising:means for computing aresult containing phase change information for each of a plurality ofpairs of said demodulated received signals including first and secondregisters for storing respective elements of said pairs of saiddemodulated received signals and a divider coupled to said first andsecond registers for dividing an element of a pair stored in said firstregister by an element of said pair stored in said second register;means for calculating the average of said results to compute the carrieroffset estimate including an averager coupled to said divider forcalculating an average of at least a subset of said results containingphase change information to compute the carrier offset estimate; and anormalizer coupled to said averager for normalizing said average tocompute the carrier offset estimate.
 10. The carrier offset estimationsystem of claim 9, further comprising:a carrier adjuster for adjustingsaid expected carrier frequency based on the carrier offset estimate,wherein said adjusted expected carrier frequency is used for subsequentreceived digital spread spectrum signals to demodulate said subsequentreceived digital spread spectrum signals to remove said carrierfrequency.
 11. The carrier offset estimation system of claim 10, furthercomprising:a validity tester for examining the validity of said carrieroffset estimate; wherein said carrier adjuster uses only valid carrieroffset estimates.